Radio frequency identification (RFID) generally employs resonance in order to increase the efficiency of energy transfer from the reader to the tag. This is achieved through the resonant recycling of energy that results in voltage step up in the tag when subject to the reader powering field. The level of step up increases as the Q of the resonant circuit increases and the loss decreases.
However, as the tag Q increases the width of the resonance drops proportionally. High levels of step up, corresponding to high Q, are only achieved over a narrow frequency band. This makes the system susceptible to degradation due to detuning as follows:                1) Environmental detuning can cause the resonant frequency to shift from the intended frequency. When read at a fixed frequency by the reader then the detuned tag may not derive sufficient power to operate.        2) Variation at manufacture of the resonator components (generally an antenna and capacitor) has a similar effect to environmental detuning and the detuned tag may not derive sufficient power to operate.        3) Mutual coupling between tags can cause their resonant frequencies to shift. The result is again that the detuned tag may not derive sufficient power to operate.        
If high levels of detuning need to be tolerated then this limits the Q of the resonator and the range of the tag decreases. Alternatively, if environmental detuning is not an issue then high Q can be used to extend the read range of the tag. However, the accuracy of the resonator components need to be high and this increases the cost of manufacture.
Auto-tuning in RFID is known to compensate for tuning differences. However, to date this has had limited application in a conventional passive tag. The tag needs to be tuned correctly in order to derive the power required to operate from the reader field. When the tag is detuned it cannot derive the power required to operate a conventional tuning circuit and therefore fails to power up.
Patents WO2007068974 and WO2008110833 outline a new method that enables auto-tuning to be used in a passive tag. Rather than being based on a conventional linear resonator, these patents introduce a non-linear resonator with beneficial tuning properties. A circuit is disclosed comprising a self-adaptive resonator that may be operated over a designed band of frequencies, independent of the level of loss. In embodiments this is achieved through the use of an antenna and two capacitive paths that are coupled into the resonance with a variable duty cycle; the duty cycle is controlled by the waveform amplitude and the gate voltage on a MOSFET. One application of this circuit is in a tag where the induced voltage is used to control the mosfet gate voltage and ramp up the amplitude in tag. This arrangement can achieve high levels of voltage step up corresponding to low loss in the antenna resonance, without the drawbacks of a narrow single resonance frequency. In effect the tag has an auto tuning behaviour to the stimulus frequency, provided it is within a designed frequency band. Furthermore, the system may be completely passive without the requirement for a separate power source to operate the tuning circuit. It therefore has application in RFID tags, allowing them to power up in the presence of a reader field, even when they are detuned.
An RFID tag needs to both power up in the presence of a reader field and also to subsequently communicate with the reader. WO2008110833 describes a set of methods for employing the new resonator in a tag. In particular, one method disclosed was to use the new resonator for the initial power up of the tag, but switching to a conventional linear resonator for communication. The advantage of this approach is that the conventional resonator is easier to control turn on and turn off when communicating from tag to reader. Also when communicating from reader to tag, it is easier to detect the envelope changes of a conventional resonator in the tag when the reader field is modulated.
Once power is derived from the reader field then a conventional tuning circuit can also be operated in the tag. The end result after a powering and conventional tuning cycle is a tag that is well tuned to the reader field and able to communicate normally. This can provide a set of benefits over standard tags as follows:                1) Increased tolerance to environmental detuning.        2) Increased tolerance to manufacturing variation        3) Increased tolerance to tag to tag detuning        4) Operation at multiple frequencies, such as 125 kHz and 134 kHz for animal ID.        
However, the use of a separate tuning circuit that is switched in instead of the self-adaptive resonator does have some drawbacks. These are:                1) A separate tuning circuit will require additional silicon area that can lead to increases in the cost of the chip used in the RFID tag.        2) The correct setting for the tuning circuit will need to be determined. For example monitoring the envelope of the resonance as a function of the tuning setting takes time, power, and increases the complexity of the silicon. This may increase the cost of the solution and also increase the time required until the tag is in a position to operate fully.        
Background prior art can be found in: WO2007/068974; WO2005/104022; and US2005/134234.
There is therefore the requirement for an RFID tag employing a self-adaptive resonator for the power up tuning and a final state of a conventionally tuned linear resonator, but without requiring a significant increase in chip complexity, chip area, and tuning time.